Immersion type electroless gold plating solutions are used for intermediate layers for the purpose of improving the solder adhesion of circuits, terminals, and so forth of printed wiring boards and improving the adhesion of reductive gold plating and the like. Most of the gold plating solutions employed for this purpose involve a toxic cyanide compound as the gold compound, but for environmental and workplace concerns, a cyanide-free gold plating solution that does not employ toxic substances have been required.
Patent applications that have been filed for cyanide-free immersion type electroless gold plating solutions include those that make use of gold sulfite compounds (see, for example, Patent Document 1 and Patent Document 2), those that make use of a salt of gold sulfites or chloroaurates (see, for example, Patent Document 3), and those that make use of gold sulfite, gold chloride, gold thiosulfate, or gold mercaptocarboxylates (see, for example, Patent Document 4). The electroless gold plating solutions described therein are cyanide free and therefore less toxic, and can be used at near neutrality, but they still have the problems of inferior solder adhesion and plating film adhesion. “Plating film adhesion” refers to the adhesion between a immersion type electroless gold plating film and the substrate and, when a immersion type electroless gold plating film is used as an intermediate layer, that refers to the adhesion between the layers above and below the film.    Patent Document 1: Japanese Patent No. 3,030,113    Patent Document 2: Japanese Patent Publication No. 2003-13249    Patent Document 3: Japanese Patent Publication No. H8-291389    Patent Document 4: Japanese Patent Publication No. H10-317157